The use of high-velocity, abrasive-laden liquid jets to precisely cut a variety of materials is well known. Briefly, a high-velocity liquid jet is first formed by compressing the liquid to an operating pressure of between 35,000 and 60,000 psi, and forcing the compressed liquid through an orifice having a diameter approximating 0.007–0.015 inch. The resulting highly coherent jet is discharged from the orifice at a velocity that approaches or exceeds the speed of sound. The liquid most frequently used to form the jet is water, and the high-velocity jet described hereinafter may accordingly be identified as a “water-jet,” a “waterjet,” or a “fluid-jet.” Numerous liquids other than water can be used without departing from the scope of the invention, and any recitation of the jet as comprising water should not be interpreted as a limitation. For example, fluids other than water can also be employed to cut materials that cannot be in contact with water. The customary terms for this process are “water-jet cutting” or “fluid-jet cutting.” This document will refer to “fluid-jet cutting” and the like not intending to exclude cutting by jets of fluid other than water.
To enhance the cutting power of the fluid-jet, abrasive materials are added to the fluid-jet stream to produce an abrasive-laden fluid-jet, typically called an “abrasive fluid-jet” or an “abrasive jet.” The abrasive fluid-jet is used to cut a wide variety of materials from exceptionally hard materials (such as tool steel, aluminum, cast-iron armor plate, certain ceramics and bullet-proof glass) to soft materials (such as lead). Abrasive fluid-jets can accomplish the cutting of intricate slots, through cuts and curves cut in metals, glass, stone, composites, and similar materials. For cutting metals, abrasive grit from a hopper at ambient air pressure is added to the fluid-jet stream prior to the impact of the jet on the workpiece. Typical abrasive materials include garnet, silica, and aluminum oxide having grit sizes ranging between approximately #36 and approximately #220.
The material forming the fluid-jet is an orifice defined in a hard jewel held in a mount. The jewel is typically a sapphire, ruby or diamond. To produce an abrasive-laden fluid-jet, the fluid-jet passes through a “mixing region” in a nozzle wherein a quantity of abrasive is entrained into the fluid-jet by the low-pressure region that surrounds the flowing liquid in accordance with the Venturi effect. The abrasive, which is under atmospheric pressure in an external hopper, is drawn into the mixing region by the lower pressure region through a conduit that communicates with abrasive contained in a hopper. The resulting abrasive-laden fluid-jet is then discharged against a workpiece through a nozzle tip that is supported closely adjacent to the workpiece.
The typical technique for cutting by fluid-jet is to mount the workpiece (sometimes also referred to as the “material being cut”) in a suitable jig, or other means for securing the workpiece, into position on an X-Y table. The fluid-jet is typically directed onto the workpiece to accomplish the desired cutting to produce a target piece having a shape and is generally under computer or robotic control. The cutting power is typically generated by means of a high-pressure pump connected to the cutting head through high-pressure tubing, hose, piping, accumulators, and filters. It is not necessary to keep the workpiece stationary and to manipulate the fluid-jet cutting tool. The workpiece can be manipulated under a stationary cutting jet, or both the fluid-jet and the workpiece can be manipulated to facilitate cutting.
A cut produced by a fluid-jet has characteristics that differ from cuts produced by traditional machining processes. Two of these characteristics include lag (also referred to as jet-lag) and taper. FIG. 1A illustrates a fluid-jet 12 cutting a workpiece 14, and a resulting lag 18 by a deflection distance L in the jet 12 in a direction opposite of jet motion 19. Every fluid-jet application is affected to some extent by the lag 18 of the fluid-jet 12 stream from a longitudinal axis 54 as the nozzle 10 moves across the workpiece 14 at a translation speed in a direction indicated by the motion 19. The faster the nozzle 10 moves, the more the fluid-jet 12 is bent by the structure of the workpiece 14 away from longitudinal axis 54. When the motion 19 of the nozzle 10 is a straight line, the fluid-jet 12 stream cuts the material of the workpiece 14 the way a wheel cutter might cut with the stream exiting the bottom of the workpiece 14 at the deflection distance L behind the place of impact 13 where the fluid-jet stream enters the workpiece. On straight cuts, the stream 12 can be moved swiftly across the workpiece 14 because the stream's deflection L is directly inline with and behind the place of impact 13, and does not affect cutting accuracy. However, on corners, the deflection 18 of the cutting-jet by the deflection distance L can cause cutting errors as it flares to the outside of a corner leaving behind or cutting undesirable deflection tapers. Also, if the jet is rapidly accelerated around a sharp corner, it may leave an uncut region and it may deflect so as to create a trough in the underside of a part 14 on the outside of the corner.
In straight-line cutting, the lag 18 is a function of the cutting-head translation speed, and a high lag causes the jet to flop from side to side resulting in a poor finish. This sets a maximum speed for the cut given a finish requirement. However, a rapid acceleration, even within the speed limit, will also cause a mark to be made on the surface. This places a constraint on the rate of change of cutting-head translation speed along the cut. Within these constraints, one wishes to move as quickly as possible to minimize the cutting time and to avoid excessive kerf caused by stopping or moving very slowly.
Every fluid-jet application is also affected by a bevel taper (also referred to as “taper”) in the cut edges of the workpiece 14. FIG. 1B illustrates a bevel taper 20 in the cut edges 17a and 17b of the workpiece 14 formed by the jet 12. The jet 12 is truncated in FIG. 1B for clarity. Jet cutting, particularly with an abrasive fluid-jet, typically produces undesirable tapered or beveled cut edges 17a and 17b in a target piece. The widest portion of the bevel taper 18 is typically toward the place of impact 13. The bevel taper 18 looks much like a sharpened end of a pencil was dragged through the workpiece 14. The bevel taper 20 is a function of material thickness, and is generally greatest in thin material where the bevel taper 20 may be 10 degrees. In thicker material such as two-inch steel, the bevel taper 20 is much less, though still significant. The bevel taper 20 is also a function of cutting speed. The bevel taper 20 becomes less as cutting speed slows, and then as cutting speed further slows beyond a point, the bevel taper 20 reverses from that illustrated in FIG. 1B becoming narrower toward the point of impact 13. The bevel taper 20 typically can only effectively be eliminated by tilting the nozzle 10 relative to the workpiece surface 15 along the X-axis. As used herein, “tilt” generally means an angle between the nozzle 10 and a plane formed by the workpiece surface 15 that is less than a normal angle. For example, tilting the nozzle 10 one degree generally means changing an orientation of the nozzle 10 from 90 degrees (normal) relative to a plane formed by the workpiece surface 15 to 89 degrees.
Unlike the bevel taper 20, the lag 18 may be reduced by slowing the motion 19 of the nozzle 10 across the workpiece 14. To cut complex shapes with a variety of corners and curves, the traverse speed of the motion 19 can be constantly adjusted, thus slowing the translation speed and increasing the cutting time. In addition, reducing undesirable deflection tapers requires that the fluid-jet 12 continue removing material from the cut surfaces 16 even after the fluid-jet 12 has penetrated the thickness of the workpiece 14. Another method of reducing undesirable deflection tapers is to make multiple passes with the fluid-jet 12 across the workpiece 14. These methods all increase time necessary to cut the workpiece 14.
Commercially viable five-axis machines have recently been developed that provide an ability to translate a fluid-jet cutting head along three orthogonal axes (X, Y and Z), and to rapidly tilt the tool about fourth and fifth orthogonal axes (referred to herein as X′ and Y′). Such a five-axis machine is described in (application Ser. No. 10/394,562), titled APPARATUS THAT HOLDS AND TILTS A MACHINE TOOL, filed Mar. 21, 2003, presently assigned to the assignee of the present invention, and which is incorporated herein for all purposes.
The ability provided by the five-axis machines to tilt the nozzle 10 relative to the workpiece surface 15 provides advantages for fluid-jet cutting. For straight-line cutting, the nozzle 10 and fluid-jet 12 can be orientated normal to the workpiece surface 15 with a compensation tilt to offset the taper 20, such as along the X-axis of FIGS. 1A and 1B. Undesirable lag in corners can be reduced by additionally tilting the nozzle 10 such that the fluid-jet is partially pointed in the direction of movement to offset lag, such as along the Y-axis of FIGS. 1A and 1B. Alternatively, the speed of the fluid-jet's movement 19 across the workpiece 14 can be maintained in a first cut with only a compensation tilt to minimize the taper 20, and then a subsequent cutting pass made across the workpiece 14 with the nozzle 10 additionally tilted to remove lag 18 produced in the previous cutting pass. This can be quicker than making one slow cutting pass that does not produce deflection tapers.
Existing cutting-head motion-control systems and methods typically have only a capacity to control movement of a cutting head along the X, Y and Z-axis, but lack a capacity to control the X′ and Y′ tilting movements of the cutting head provided by the five-axis machines. In view of the foregoing, there is a need in the art for a new and improved system and method for controlling tilting movements of the cutting head provided by the five-axis machines.